Mass airflow information required for precise fueling of an automotive internal combustion engine is commonly obtained with a mass airflow sensor installed upstream from the intake manifold of the engine. In order to avoid error due to momentary flow reversals in the manifold, the sensor preferably measures both in-flow and out-flow; that is, the sensor must be bi-directional.
A well known bidirectional sensor topology comprises two temperature sensor elements disposed in-line along the intake air stream, and a heater element disposed between the two sensor elements. Airflow in the manifold is detected as a function of the difference between the temperatures at the two sensor locations. When there is no flow, equal amounts of heat from the heater element reach both sensor elements, and no temperature difference is detected. With an air in-flow or out-flow, one of the sensors (the downstream sensor) is heated more than the other sensor (the upstream sensor), resulting in a sensed temperature difference that varies monotonically with airflow. The temperature sensors are typically connected with a pair of external precision resistors to form a Wheatstone bridge circuit to convert the sensed temperature difference into a corresponding voltage. Representative sensor configurations of this type are shown and described in the U.S. Pat. Nos. 4,576,050, 5,243,858, 5,263,380, 5,629,481 and 5,631,417, all of which are assigned to the assignee of the present invention.
While the above-described sensor topology is advantageous in many respects including low cost of manufacture and small package size, it tends to exhibit inherent limitations in sensitivity and accuracy due to the bridge characteristics, parasitic resistance and mismatches in the temperature coefficient of resistance (TCR) of the on-chip and external resistances. While these limitations can be compensated to a certain extent through the use of high gain amplification and exotic materials, these techniques add significantly to the cost of the sensor. What is desired is a new sensing device that retains the packaging and low cost advantages and the fast r esponse time and wide bandwidth of the above-described sensor topology while overcoming these performance limitations.